The invention relates to a method and device for controlling the mode of combustion of an internal combustion engine, and in particular a four-stroke petrol engine.
In a controlled-ignition four-stroke petrol engine:
the fuel is injected directly into the combustion chamber via high-pressure fuel supply system;
the control parameters are calculated and applied by a central control unit;
the exhaust gases are treated by one or more catalytic converters placed in the exhaust line.
The engine control unit processes the various demands on the engine (the wishes of the driver, the on-board electronic systems of the course-control or gearbox type, etc.), assimilates them and produces a reference torque value to be achieved by action on the control parameters which are:
the air flow rate
the amount of fuel injected
the ignition advance applied.
For a direct-injection petrol engine, the engine control unit has various modes of combustion available in order to achieve this reference torque value.
At each instant it has to evaluate the mode of combustion that will produce the best fuel consumption/driveability/exhaust gases clean-up compromise.
One of the fundamental characteristics of the modes of combustion is the richness of the air/fuel mixture that they allow.
The richness of the mixture is a dimensionless quantity defined as the ratio between the air/fuel proportions of a stoichiometric mixture and the same air/fuel proportion of the mixture in the mode of combustion considered.
xe2x80x83Richness for the mode of combustion i=(airflow rate/fuelflow rate) stoichiometric/(airflow rate/fuelflow rate)mode i.
By definition:
the richness is equal to 1 when the mixture is stoichiometric,
the richness is greater than 1 when the proportion of petrol in the mixture is greater than that of the stoichiometric mixture. The mixture is said to be xe2x80x9crichxe2x80x9d,
the richness is less than 1 when the proportion of petrol in the mixture is lower than that of the stoichiometric mixture. The mixture is said to be xe2x80x9cleanxe2x80x9d.
The mode of combustion which provides the best efficiency is the so-called xe2x80x9cstratified chargexe2x80x9d mode.
In this mode, the fuel is injected into the combustion chamber at the end of the compression phase so that the richness of the mixture near the spark plug at the time of ignition is high enough to ensure combustion.
The overall mixture has a very great excess of air (mean richness of the order of 0.4) which allows:
an increase in the combustion efficiency of the engine,
an increase in the mean pressure in the intake manifold and thus a reduction in xe2x80x9cpumping lossesxe2x80x9d.
The range of use of this mode of combustion is physically restricted by the maximum air-filling of the cylinders, which filling is associated with the maximum pressure in the plenum chamber (full air charge).
This xe2x80x9cstratified chargexe2x80x9d mode of combustion is therefore favoured for low torque demands but cannot meet all the demands placed on the engine by the driver.
For higher torque demands, there are two modes of combustion that may be employed, both characterized by the injection of fuel into the chamber during the inlet phase.
This injection allows homogenous mixing of air and fuel.
The distinction between these two xe2x80x9chomogenousxe2x80x9d modes of combustion is in the associated mean richness level:
Lean Homogenous Mode
The mean richness of the mixture is of the order of 0.75.
This mode has the same advantages of the aforementioned stratified charge mode, limited by the overall richness level which has to be high enough to allow combustion of the mixture.
Stoichiometric Homogeneous Mode
The richness of the mixture is equal to 1.
This mode is needed for high demands for engine torque, which demands require high fuel flow rates.
A rich homogeneous mode is also defined for when the engine is running at full load. This mode will not be mentioned here because it is not specific and can be likened to the stoichiometric homogenous mode for the aspects dealt with.
The favoured mode of combustion for optimizing consumption can be represented schematically by the graph of engine torque against engine rotational speed as shown in FIG. 1.
The transition from one mode of combustion to another is to be made with no appreciable affect as far as the driver is concerned.
This constraint entails complex management of the actuators by the engine control unit.
For example, without any specific action by the control unit, the change from one mode of operation with a lean mixture to the homogenous stoichiometric mode of operation would give rise to a sudden and significant increase in torque, which needs to be avoided.
To do this, the engine control unit calculates the air, fuel and ignition advance controls in order at every instant to meet the reference torque value.
This control xe2x80x9cby torquexe2x80x9d makes it possible to ensure a torque which is equal to the demand from the driver, including in changes of mode.
However, the quality with which the reference torque value is adhered to is dependent on spread which may be brought about by the ageing of engine parts, manufacturing spread or even the varying characteristics of the commercially available fuels.
These variations carry the risk of disrupting the control by torque and thus of making the changes in mode perceptible to the user.
It is therefore important that changes in mode of combustion should be brought about only for lasting changes in the engine operating point.
The influence of the mode of combustion on emissions of pollutants will now be discussed.
Emissions or pollutants from the engine are treated by a catalytic system that forms cart of the exhaust system.
This system may be made up of one or more elements intended to oxidize or to reduce the toxic components of the exhaust gases.
The most dangerous components are unburnt hydrocarbons (HC), carbon monoxide (CO) and the oxides of nitrogen (NOx).
The CO and the HCs need to be oxidized to convert them into CO2+H2O.
The NOx have to be reduced to be converted into H2+O2.
When the air-petrol mixture is stoichiometric, the dual function or oxidation and reduction is carried out by a three-way catalytic converter.
When associated with fine adjustment of the richness of the air-fuel mixture allowing the richness to be made to fluctuate by small amounts around 1, this catalytic converter allows excellent overall conversion of the two pollutants.
When the air-petrol mixture is lean, only the oxidation function can be performed by the three-way catalytic converter.
The reduction function can then be provided in various ways:
NOx can be stored while the mixture is lean and then reduced during phases in which the engine is operating at a richness greater than or equal to 1,
chemical formulation can be used to allow a reduction in lean mixtures.
Whatever the definition of the catalytic system, its treatment efficiency is very low for as long as its temperature has not reached a threshold light-off temperature at which the chemical reactions begin.
As long as this light-off temperature (of the order of 250xc2x0) has not been reached, specific engine management is required in order to:
minimize basic engine emissions as far as possible,
increase the temperature of the catalytic system as quickly as possible.
This management favours the clean-up constraint at the expense of fuel consumption. It has therefore to be suppressed as soon as the control unit detects sufficient efficiency for the pollutants to be converted in the mode of combustion that favours consumption.
The invention aims to create a method and a device for the overall management of the constraints associated with the choice of combustion mode.
The constraints taken into consideration are the fuel consumption, the driveability of the vehicle and the efficiency with which the pollutants are treated as the temperature rises after the engine has been started.
A subject of the invention is therefore a method for controlling the mode of combustion of a controlled-ignition four-stroke petrol engine equipped with a system for the direct injection of fuel into the combustion chamber, with at least one catalytic converter placed in the exhaust line of the engine and with a control system receiving information relating to the rotational speed and to the load of the engine, to the position of the accelerator pedal, and to the temperatures of the engine and of the exhaust gases, characterized in that an estimate of the combustion efficiency of the various modes available is established and, bearing in mind the said information relating to the rotational speed and to the load of the engine, to the position of the accelerator pedal and to the temperatures of the engine and of the exhaust gases, a priority mode of combustion is chosen on the basis of the said estimate of the combustion efficiency of the various modes available.
Another subject of the invention is a device for controlling the mode of combustion of a controlled-ignition four-stroke petrol engine equipped with a system for the direct injection of fuel into the combustion chamber, with at least one catalytic converter placed in the exhaust line of the engine and with a control system receiving, from sensors, information relating to the rotational speed and to the load of the engine, to the position of the accelerator pedal, and to the temperatures of the engine and of the exhaust gases, for implementing the method defined hereinabove, characterized in that the control system comprises means or choosing a priority mode of combustion taking account of the said information and on the basis of an estimate of the combustion efficiency of the various modes available.
According to other features:
the device comprises a control algorithm making it possible to calculate the combustion efficiency taking account of the thermal condition of the combustion chamber;
the control algorithm makes it possible to correct the priority mode of combustion using a switching efficiency capable of anticipating the behaviour of the driver and thus of avoiding ill-timed changes in combustion mode upon discrete changes of priority mode of combustion;
the control algorithm makes it possible to anticipate the behaviour of the driver on the basis of a combined analysis of engine torque reference values before and after the application of filters which are intended to smooth out the transitions in torque in order to make the vehicle pleasant to drive;
the control algorithm makes it possible to correct the mode of combustion taking account of the treatment efficiency of the said at least one catalytic element in the exhaust line as the temperature rises after the engine has been started.
Constraint management is prioritized as follows:
a priority mode of combustion is defined by the minimum consumption criterion.
The performance of a mode of combustion is expressed in the form of a combustion efficiency and the mode of combustion that gives the best efficiency is chosen as the priority mode.
if this priority mode changes, the control unit tests the stability of the new mode over time. The purpose of this test is to detect discrete changes which must not be applied because they might have an effect on the torque that the driver can perceive. Discrete chances are detected by anticipating the behaviour of the driver.
This anticipation is made possible by filtering the wishes of the driver, which filtering is constantly in use in order to smooth the torque demand and thus to guarantee good driveability of the vehicle.
Comparing reference torque values before and after filtering makes it possible to discriminate between mode changes which have to be applied without a time delay and those which must not be applied.
The mode of combustion which takes account of the consumption and driveability constraints is finally compared against the clean-up constraint imposed by the catalysis system.
After the engine has been started, the engine control unit evaluates the efficiency of the treatment of pollutants by the catalysis system.
This estimate of the efficiency, expressed in the form of a conversion efficiency, allows the control unit to impose, during the temperature rise, the mode of combustion which guarantees the lowest level of emissions of pollutants.
Once the nominal operating temperature has been reached, this constraint is taken away and the priority mode of combustion is authorized.